The present invention relates to a method for the manufacture of bundles of metal sheets of annular shape for stators of electric machines from stamped sheets according to the following steps: rectilinear sheet segments are cut out by stamping, with a length equal to the length of the periphery of the annular bundle to be formed, each segment taking the shape of a band, the teeth of which have the shape of the radial cross section of the pole pieces of the stator and the side opposite the teeth has notches arranged in the extension of the teeth, the segments are stacked to form a bundle of predetermined height, and they are assembled by means of at least two laser-welding or plasma-welding beads in two grooves formed by the notches. The invention also relates to a plant for carrying out the method.
It is known, for constructing stators of small and medium electric machines and, in particular, of alternators of motor vehicles, to form bundles of rings cut out by stamping in magnetic sheets or the like and to assemble them by riveting or welding.
The cut-out rings have, toward the inside, open slots formed by arms which are directed radially inward and which in the bundle will form the pole pieces of the stator. Cutting out an entire ring by stamping has some disadvantages: there is a large amount of waste, since that part of the sheet which corresponds to the inside diameter of the ring can be used only for forming a ring of smaller diameter, and indeed only insofar as the thickness of the sheet and the diameter correspond to the technical characteristics required. On the other hand, there is also a large amount of sheet waste corresponding to that part of the sheet which is outside the diameter of the ring.
The construction of the rings does not make it possible to maintain the required shape tolerances and the exact dimensions both of the rings and of the ring bundles on account of the internal stresses which are generated during stamping and which cannot easily be eliminated because of the closed shape of the ring. The internal stresses have an influence on the size tolerances of the inside and outside diameters of the rings and on the parallelism of two faces of a sheet bundle. It is often necessary to carry out additional machining of the stator before the required tolerances can be adhered to.
The Applicant""s U.S. Pat. No. 4,597,172, the content of which is incorporated by reference, proposed a method and a plant for the manufacture of bundles of metal sheets from ring segments, in particular ring segments at 120xc2x0, without this being a limiting value.
This method and apparatus are satisfactory and make it possible, on the one hand, to make savings with regard to the sheet waste, since only ring segments are cut out and, on the other hand, to adhere more closely to the geometry of the bundle in that the internal stresses generated during the stamping of a closed ring are eliminated.
U.S. Pat. No. 4,102,040, the content of which is incorporated by reference, proposes a manufacturing method which, according to one variant, involves bending the sheet bundle formed by causing it to mate with the lateral surface of a drum in a continuous operation. When the two ends of the bundle meet, they are held by means of a connecting device and welding is then carried out.
U.S. Pat. No. 4,198,743, the content of which is incorporated by reference, describes a method which, according to one variant, makes it possible to form a C-shaped magnetic core by the middle of a sheet bundle being immobilized and by two simultaneously gripping tools being employed to exert, from the center, continuous forces making it possible to obtain the C-shape.
The object of the present invention is to propose a new method for the manufacture of bundles of metal sheets for the stators of electric machines, making it possible, in particular, to reduce considerably the waste attributable to this manufacture and to simplify the implementing plant.
The method according to the present invention is defined by a combination of the following steps:
The bundle is held gripped between a flat surface bearing on the bundle surface having the grooves, over approximately half the length of the segment, leaving approximately one quarter of the total length free at each end, and means bearing on the face having the pole pieces, the two ends are prebent by means of a multiple-slide press over approximately half their free parts, so as to form, on each of them, an arc of approximately 40xc2x0, by pressure being applied to their corresponding grooved surfaces, against punches of corresponding shape, the two free ends are bent by means of a multiple-slide press over the entirety of their free parts in a 90xc2x0 arc of a circle, by pressure being applied to the grooved surface of the two ends, against punches of corresponding shape, the bundle is released and is held solely along two edges, one on the inner face and the other on the outer face of the bundle, corresponding to the midpoints of the segments, pressure is applied to the arcuate parts of the bundle until the cylinder is closed, the joined ends are laser-welded or plasma-welded, thus forming a cylindrical component.
The advantages of the method according to the invention are that the formation of the ring is carried out sequentially, thus allowing better cylindricity due to the fact that the fibers of the metal expand between two steps. On the other hand, the machine used is a multiple-slide machine which has existed for a long time. It is necessary to adapt the machine to implement the method with the result being that there is virtually no need for correction of cylindricity after the welding of the ends of the folded bundle.
According to a preferred variant of the invention, two segments are cut out from a sheet metal band, which are back to back and are offset at a distance equal to half the distance between two pole pieces, thus making it possible to limit sheet waste to a minimum, virtually to those parts of the sheet which correspond to the spaces between the poles.
On the other hand, there is no stress during stamping, since the pieces are cut out in straight segments, thus avoiding the problems which they may present with regard to the geometry of the stator.
In order to adhere to the geometry of the stator, the sheet bundles, after being finally assembled, may be subjected to calibration by means of a hydraulic press in the case of strict tolerances.
A plant for implementing the method according to the invention is also proposed.
The implementing plant is defined in that it comprises a sheet metal stamping station for the formation of rectilinear segments, a station for stacking and deforming the bundles with a predetermined height, a station for assembling by the affixation of at least two welding beads, and a station for folding the bundles on a multiple-slide press to form cylindrical components, said folding station likewise being provided with a station for welding the joined ends of each bundle, the folding station comprising a first plate provided with a bearing surface perpendicular to said plate, for receiving each of the bundles, and with three slides, one which of is located on the side having the teeth and is provided with a stop piece for holding the bundle against said bearing surface and two punches and two slides are located on the opposite side, each provided with a die for bearing on the ends of the bundle and for prebending them over approximately 40xc2x0 against the punches of the first slide, with a second plate parallel to the first and having three slides arranged in the same way as those of the first plate, the two slides acting on the outer surface of the bundle having dies with a surface concave over approximately a quarter of a cylinder, in order to push the prebent ends of the bundle against corresponding punches of the opposite slide into an arc of a circle of 90xc2x0, with a third plate provided with a bearing surface for receiving the still planar part of the previously bent bundle, with a slide provided with a stop surface for holding this bundle against said flat surface and two slides facing one another on either side of the 90xc2x0-bent parts of the bundle and provided with dies having a concave cylindrical surface intended for pushing the arcuate ends of the bundle up to complete closure and the formation of a cylinder, and with a fourth plate provided with a bearing surface, on which the bundle rests with its generatrix opposite the generatrix along which the two ends of the bundle are joined, with a first slide provided with a member making it possible to hold the bundle against said bearing surface and with four slides uniformly distributed along a circumference and provided with four dies with concave surfaces having the dimensions and the radius of approximately a quarter of a cylinder, making it possible to keep the ring thus formed closed, and with a laser-welding or plasma-welding device for welding the joined ends of the bundle along a generatrix.